You know those tables you were told about flipping in the tutorial?
┬─┬ノ( º _ ºノ)
Flip 500 of them!
You should EASILY get this as you grind for the other achievements, so there is no need to specifically grind for this. However, if you want to get this out the way for whatever reason...
┻━┻ ︵ヽ(`Д´)ﾉ︵ ┻━┻
Flip every table you see! You will see the most tables on the first floor, so you can technically keep restarting before you enter the boss chamber and knock this out.